1. Field of the Invention
The present invention relates to optical communication equipment and, more specifically, to optical receivers.
2. Description of the Related Art
In heterodyne detection, a relatively weak communication signal is mixed with a relatively strong local oscillator (LO) signal having a frequency that is sufficiently close to the frequency of the communication signal to result in coherent phase interference. Due to said interference, the communication and LO signals mix to produce an intermediate-frequency (IF) signal. The IF signal carries the same information as the communication signal, but has a frequency that is equal to the frequency difference between the communication and LO signals. The power of the IF signal is proportional to the product of the amplitudes of the communication and LO signals. Therefore, when the amplitude of the local oscillator signal exceeds the amplitude of the communication signal, heterodyne detection provides signal amplification with respect to direct detection of the communication signal.
FIG. 1 shows a block diagram of a representative prior-art optical heterodyne receiver 100 having an optical-to-electrical (O/E) signal converter 130 coupled to a signal decoder 140. An optical communication signal 102 applied to O/E converter 130 and a continuous-wave LO signal 106 generated by a local oscillator (e.g., a laser) 104 are mixed in an optical coupler 108 to produce two mixed signals 110a and 110b preferably having a relative phase shift of 180 degrees. Each of mixed signals 110a-b includes an IF component as well as additional components at the frequencies corresponding to the wavelengths of signals 102 and 106. Each of mixed signals 110a-b is detected by a corresponding photo-detector (e.g., a photodiode) 112, which, due to its bandwidth limitations, also serves as a low-pass filter. As a result, the additional components are filtered out by photo-detectors 112a-b, and electrical signals 114a-b generated by the photo-detectors represent the IF components of mixed signals 110a-b, respectively. Each of signals 114a-b is coupled, via an amplifier 116 and a variable attenuator 118, to a corresponding input of a differential amplifier 120. Amplifiers 116 and variable attenuators 118 serve to balance signals 114a-b such that these signals have equal amplitudes at the inputs of differential amplifier 120. Signal 122 generated by differential amplifier 120 is then processed in signal decoder 140 to recover the data carried by communication signal 102. Signal decoder 140 is adapted to (i) sample signal 122 one time per signaling interval, (ii) based on the signal sample, determine the value of a corresponding communication data bit, and (iii) output the determined value into a bit stream 162. A more detailed description of receiver 100 is provided in U.S. Pat. No. 4,718,121 , the teachings of which are incorporated herein by reference.
When used in a wavelength-division multiplexing (WDM) communication system, receiver 100 locks onto a selected WDM channel by appropriately tuning the output wavelength of local oscillator 104. Local oscillator 104 is typically a distributed-feedback (DFB) laser controlled by temperature and/or injection current. Currently, DFB lasers are capable of reproducing a selected wavelength with an accuracy of only about 0.01 to 0.1 nm. Due to wavelength errors inherent to DFB lasers, the LO frequency deviates from the value prescribed for the selected WDM channel and some additional fine wavelength tuning of the DFB laser is usually required to better reproduce the selected wavelength and reduce the number of decoding errors in signal decoder 140 induced by the initial wavelength error. The dashed line in FIG. 1 indicates a feedback line that enables this fine wavelength tuning. Disadvantageously, the fine wavelength tuning significantly increases the channel-switching time in a WDM receiver.